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Don’t be a Nutrition Statistic: Vitamin D, Bones, and More

By Michael McBurney

The first identified biological role of vitamin D in calcium absorption and deposition. Of course, calcium is also essential to build bones and teeth. Because of the complex regulation of vitamin D and its metabolites, researchers have wondered if our bodies might conserve vitamin D during periods of low calcium intake. If so, vitamin D requirements could be affected by dietary calcium intake.

Cashman and colleagues conducted a 15wk, randomized placebo-controlled double-blind (RCT) study with 125 free-living men and women (≥50y). They were stratified to two calcium intakes (<700 and >1000 mg/d) and either placebo or 800 IU vitamin D3 daily. The study was conducted in Ireland during the winter months to minimize endogenous vitamin D contributions from skin exposure to sunlight. Serum 25(OH)D concentration, and those of several metabolites of vitamin D, were affected by vitamin D3 supplementation but not by calcium intake. While extreme calcium restriction might have an effect on vitamin D catabolism, the authors conclude that vitamin D requirements are not affected by relatively low or high calcium intakes.

During the winter months, serum 25(OH)D levels fall. Cashman and associates recruited men and women with average baseline serum 25(OH)D concentrations ranging between 54-58 nmol/L. In those receiving placebo supplements, serum 25(OH)D concentrations fell ~25% over the next 15 weeks of winter whereas those taking vitamin D increased to 74-80 nmol/L. Similarly, 84% of New Zealand participants had suboptimal vitamin D concentrations (<50 nmol/L) when taking a placebo over winter months.

Vitamin D is important for health. In a recent meta-analysis, Chowdhury and colleagues report that every 25 nmol/L drop in serum 25(OH)D concentration is associated with a 16% increased risk of mortality. Statistically, the difference between maintaining serum 25(OH)D concentrations at 75 nmol/L vs 25 nmol/L is a 32% difference in risk of dying. For more on the two Apr 1, 2014 British Medical Journal publications on vitamin D, read yesterday’s blog.